Wnt/β‐catenin‐mediated transcriptional regulation

Distinct and mutually inhibitory binding by two divergent β-catenins coordinates TCF levels and activity in C. elegans

Wnt target gene activation in C. elegans requires simultaneous elevation of β-catenin/SYS-1 and reduction of TCF/POP-1 nuclear levels within the same signal-responsive cell. SYS-1 binds to the conserved N-terminal β-catenin-binding domain (CBD) of POP-1 and functions as a transcriptional co-activator. Phosphorylation of POP-1 by LIT-1, the C. elegans Nemo-like kinase homolog, promotes POP-1 nuclear export and is the main mechanism by which POP-1 nuclear levels are lowered. We present a mechanism whereby SYS-1 and POP-1 nuclear levels are regulated in opposite directions, despite the fact that the two proteins physically interact. We show that the C terminus of POP-1 is essential for LIT-1 phosphorylation and is specifically bound by the diverged β-catenin WRM-1. WRM-1 does not bind to the CBD of POP-1, nor does SYS-1 bind to the C-terminal domain. Furthermore, binding of WRM-1 to the POP-1 C terminus is mutually inhibitory with SYS-1 binding at the CBD. Computer modeling provides a structural explanation for the specificity in WRM-1 and SYS-1 binding to POP-1. Finally, WRM-1 exhibits two independent and distinct molecular functions that are novel for β-catenins: WRM-1 serves both as the substrate-binding subunit and an obligate regulatory subunit for the LIT-1 kinase. Mutual inhibitory binding would result in two populations of POP-1: one bound by WRM-1 that is LIT-1 phosphorylated and exported from the nucleus, and another, bound by SYS-1, that remains in the nucleus and transcriptionally activates Wnt target genes. These studies could provide novel insights into cancers arising from aberrant Wnt activation.

The chromatin remodelers ISWI and ACF1 directly repress Wingless transcriptional targets

The highly conserved Wingless/Wnt signaling pathway controls many developmental processes by regulating the expression of target genes, most often through members of the TCF family of DNA-binding proteins. In the absence of signaling, many of these targets are silenced, by mechanisms involving TCFs that are not fully understood. Here we report that the chromatin remodeling proteins ISWI and ACF1 are required for basal repression of WG target genes in Drosophila. This regulation is not due to global repression by ISWI and ACF1 and is distinct from their previously reported role in chromatin assembly. While ISWI is localized to the same regions of Wingless target gene chromatin as TCF, we find that ACF1 binds much more broadly to target loci. This broad distribution of ACF1 is dependent on ISWI. ISWI and ACF1 are required for TCF binding to chromatin, while a TCF-independent role of ISWI-ACF1 in repression of Wingless targets is also observed. Finally, we show that Wingless signaling reduces ACF1 binding to WG targets, and ISWI and ACF1 regulate repression by antagonizing histone H4 acetylation. Our results argue that WG signaling activates target gene expression partly by overcoming the chromatin barrier maintained by ISWI and ACF1.

Regulation of the feedback antagonist naked cuticle by Wingless signaling

Signaling pathways usually activate transcriptional targets in a cell type-specific manner. Notable exceptions are pathway-specific feedback antagonists, which serve to restrict the range or duration of the signal. These factors are often activated by their respective pathways in a broad array of cell types. For example, the Wnt ligand Wingless (Wg) activates the naked cuticle (nkd) gene in all tissues examined throughout Drosophila development. How does the nkd gene respond in such an unrestricted manner to Wg signaling? Analysis in cell culture revealed regions of the nkd locus that contain Wg response elements (WREs) that are directly activated by the pathway via the transcription factor TCF. In flies, Wg signaling activates these WREs in multiple tissues, in distinct but overlapping patterns. These WREs are necessary and largely sufficient for nkd expression in late stage larval tissues, but only contribute to part of the embryonic expression pattern of nkd. These results demonstrate that nkd responsiveness to Wg signaling is achieved by several WREs which are broadly (but not universally) activated by the pathway. The existence of several WREs in the nkd locus may have been necessary to allow the Wg signaling-Nkd feedback circuit to remain intact as Wg expression diversified during animal evolution.

Wingless signaling induces widespread chromatin remodeling of target loci

How signaling cascades influence gene regulation at the level of chromatin modification is not well understood. We studied this process using the Wingless/Wnt pathway in Drosophila. When cells sense Wingless ligand, Armadillo (the fly beta-catenin) becomes stabilized and translocates to the nucleus, where it binds to the sequence-specific DNA binding protein TCF to activate transcription of target genes. Here, we show that Wingless signaling induces TCF and Armadillo recruitment to a select subset of TCF binding site clusters that act as Wingless response elements. Despite this localized TCF/Armadillo recruitment, histones are acetylated over a wide region (up to 30 kb) surrounding the Wingless response elements in response to pathway activation. This widespread histone acetylation occurs independently of transcription. In contrast to Wingless targets, other active genes not regulated by the pathway display sharp acetylation peaks centered on their core promoters. Widespread acetylation of Wingless targets is dependent upon CBP, a histone acetyltransferase known to bind to Armadillo and is correlated with activation of target gene expression. These data suggest that pathway activation induces localized recruitment of TCF/Armadillo/CBP to Wingless response elements, leading to widespread histone acetylation of target loci prior to transcriptional activation.

CBP/p300 are bimodal regulators of Wnt signaling

Many Wnts influence cell behavior by a conserved signaling cascade that promotes the stabilization and nuclear accumulation of beta-catenin (beta-cat), which then associates with TCF family members to activate target genes. The histone acetyltransferase CREB binding protein (CBP) can bind to TCF and inhibit Wnt signaling in Drosophila. In contrast, studies in vertebrates indicate a positive role for CBP and the closely related protein p300 as beta-cat binding transcriptional co-activators. We address this discrepancy by demonstrating that in addition to its negative role, CBP has an essential positive role in Wnt signaling in flies. CBP binds directly to the C-terminus of Armadillo (Arm, the fly beta-cat) and is recruited to a Wnt-regulated enhancer (WRE) in a Wnt- and Arm-dependent manner. In a human colorectal cancer cell line, we show that CBP and p300 can inhibit Wnt signaling and demonstrate that human p300 can bind directly to TCF4 in vitro. Our results argue that CBP/p300 has an evolutionarily conserved role as a buffer regulating TCF-beta-cat/Arm binding. Subsequent to this interaction, it also has an essential role in mediating the transactivation activity of beta-cat/Arm.